1. Field of the Invention
The present invention relates to an optical ring architectures in general and, more particularly, to an optical node system for an optical ring architecture.
2. Description of the Related Art
Fiber optic systems have increasingly taken over the functions of their copper counterparts in the trunk network and between central offices due to their inherent low loss and high bandwidth. A typical central office ring configuration 100 is depicted in FIG. 1 and includes a plurality of central offices 101a-d. Each central office (CO) 101a-d is capable of transmitting calls to any of the other COs, either directly via direct connections 103a-d or indirectly through other COs as shown by logical connections 105a-b.
Typically, a CO 101a includes an add/drop multiplexer (not shown) which adds calls to the ring 103a-d destined for another CO 101b-d or drops calls from the ring 103a-d for the CO 101a.
FIG. 2 depicts a more detailed diagram of a ring architecture 106 of COs 101a-c. As shown, each CO 101a-c is capable of receiving and transmitting information over a plurality of optical fibers 107. Typically, each fiber 107 operates at a predetermined optical wavelength or wavelength band, but a single optical fiber 107 could carry traffic having multiple wavelengths. The optical fibers 107 can be unidirectional in either direction or bidirectional. Some of the optical signals on the optical fibers 107 will terminate at the CO 101a-c in that terminal equipment (not shown) in the CO 101a-c converts the optical signal to electronic form, while other optical signals will continue through the ring 106. In a typical example, an optical signal on a path 107 for destination equipment 110 is "dropped" from the ring by an add/drop multiplexer (not shown) in the CO 101a to terminal equipment (not shown) in the CO 101a. The terminal equipment (not shown) in the CO 101a may convert the optical signal to an electrical signal and pass the electrical signal along path 112 to destination equipment 110. The path 112 could be copper lines, and the destination equipment 110 is typically a terminal.
Calls can be "added" to the ring from source equipment for destination equipment 118. Source equipment 114 produces a signal along path 116 to the CO 101a. The path 116 could be copper lines for carrying electrical signals. In the CO 101a, the terminal equipment (not shown) receives the electrical signal and converts it to an optical signal. The add/drop multiplexer (not shown) in the CO 101a receives the optical signal and adds it onto a path 107. The optical signal added is routed along one or more of the paths 107 interconnecting one or more of the other COs 101a-c and is eventually "dropped," for instance, by an add/drop multiplexer (not shown) of the CO 101c which is connected to the destination equipment 118. The add/drop multiplexer (not shown) passes the optical signal to terminal equipment (not shown) in the CO 101c. As described above, the terminal equipment (not shown) in the CO 101c passes the signal along path 120 to the destination equipment 118. Each of the optical signals not being dropped at a particular CO 101a-c can be amplified and passed along to the next CO.
FIG. 3 shows a more detailed diagram of a CO 130 in a ring architecture. A CO 132 transmits over a path 134 a plurality of optical signals as a wavelength division multiplexed optical signal with wavelengths .lambda..sub.1 . . . .lambda..sub.n. Instead of using spatial multiplexing where one wavelength is in each fiber, wavelength division multiplexing (WDM) can increase capacity or decrease costs because multiple wavelengths can be put on a single fiber. The optical signal is received by the CO 130, and a pre-amplifier 138 might be used to amplify the optical signal. In particular, an erbium-doped fiber amplifier (EDFA) can be used to simultaneously amplify all of the optical signals having a plurality of wavelengths in a linear fashion. In this particular embodiment, an add/drop multiplexer 140 receives the optical signal. The add/drop multiplexer 140 includes wavelength selection devices 142 and 144, such as a wavelength grating routers (WGRs). An example of a WGR is disclosed in "Integrated Optics N.times.N Multiplexer On Silicon", Dragone et al., IEEE Phot. Technol. Lett., Vol. 3, pages 896-899 (1991).
The WGR 142 routes the incoming optical signals as a function of wavelength, to a particular output port of the WGR 142. For example, an optical signal at a wavelength of .lambda..sub.1 applied over the path 134 to WGR 142 is routed by the WGR 142 to path 146. Moreover, an optical signal at a wavelength of .lambda..sub.2 applied over the path 134 to the WGR 142 is routed by the WGR 142 to path 148. Optical signals having particular wavelengths can be "dropped" by the WGR 142. In this particular example, an optical signal having a particular wavelength .lambda..sub.n is routed onto path 150 by the WGR 142 and thereby dropped to terminal equipment 152. The terminal equipment 152 includes a receiver 156 that receives the optical signal from the path 150 and converts the optical signal to an electrical signal, thereby terminating the optical path for that particular wavelength. The receiver 156 outputs the electrical signal to electrical circuitry 158 for routing the electrical signal to the proper destination equipment 160. The electrical circuitry 158 can include a host digital terminal, switches and other electronic processors and circuitry. The destination equipment 160 can include subscriber telephones 162a-b, remote terminal equipment 164 connected to subscriber telephones 162c-d, or other local data networks.
If a call is placed by a subscriber telephone 162a, the electrical signal representing the call passes over path 166 to the CO 130. At the CO 130, the electrical circuitry 158 processes the call and sends the electrical signal to transmitter 170. The transmitter 170 outputs an optical signal having the wavelength .lambda..sub.n by using the electrical signal to modulate a laser that can produce light having the wavelength .lambda..sub.n. The transmitter 170 transmits the optical signal via path 172 to WGR 144, which multiplexes the optical signal onto the wavelength division multiplexed signal on path 174. On the path 174, the optical signal can be amplified by amplifier 176, such as an EDFA, before being output from the CO 130.
In current ring architectures, optical fibers corresponding to optical signals dropped at an add/drop multiplexer on the ring terminate at the add/drop multiplexer. For example, in FIG. 3, if an optical signal having wavelength .lambda..sub.n is dropped at the add/drop multiplexer 140, the optical fiber terminates at the terminal equipment 152 where the optical signal is converted to an electrical signal. For the add/drop multiplexer 140 to add the optical signal from the path 172 onto the path 174, the transmitter 170 must provide light having wavelength .lambda..sub.n from an optical source. As such, a relatively costly and accurately tuned laser and its supporting electronics is used for each wavelength of optical signals dropped at each add/drop node on the ring. Thus, current ring architectures can be costly and inflexible.
Accordingly, a node configuration is needed for a more flexible ring architecture which reduces costs associated with current ring architectures.